Method and apparatus for monitoring the condition of plasma equipment

ABSTRACT

An apparatus ( 100 ) senses a degree of cleanliness of a plasma reactor having a chamber ( 102 ) containing a plasma ( 103 ) that emits light ( 104 ) during a process conducted in the chamber ( 102 ). The apparatus ( 100 ) also has a light sensing element ( 180 ), configured to sense an intensity of the light ( 104 ) emitted by the plasma ( 103 ) after the light ( 104 ) passes through a film ( 135 ) that accrues in the chamber ( 102 ) during the process, and to provide a light intensity indication signal, and an electronics assembly ( 170 ) configured to receive the light intensity indication signal and to provide an indication of the degree of cleanliness of the plasma reactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to and claims priority to U.S.provisional application Ser. No. 60/307,174 filed Jul. 24, 2001, theentire contents of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to sensors and sensing methods.More specifically, the invention relates to methods and apparatus forsensing and monitoring the accumulation of impurities or other filminside a process chamber.

[0004] 2. Description of the Background

[0005] Manufacturers of semiconductor integrated circuits (ICs) arefaced with intense competitive pressure to improve their products and asa result, pressure to improve the processes used to fabricate thoseproducts. This pressure in turn is driving the manufacturers of theequipment used by IC manufacturers to improve the performance of theirequipment, and in particular to reduce the operating cost to users oftheir equipment.

[0006] One particular type of tool that is widely used, and is thereforeparticularly susceptible to these competitive pressures, is the plasmareactor. These reactors are used to remove material from a wafer by aprocess called plasma etching. The mechanisms of plasma etching arecomplex and it is essential to maintain and control the plasmaparameters and chamber conditions. Maintenance and monitoring of plasmaprocess chamber conditions is the focus of significant technologicaldevelopment in the industry.

[0007] One of the key factors affecting product quality and theproductivity of plasma processes is the presence of defect formingparticles and related contamination within a chamber within a plasmareactor. The accumulation of polymers and other byproducts of the etchprocess from process chamber components causes yields to drop andmaintenance expenses to increase. The manufacturer of plasma reactorsthat can address these issues and consistently demonstrate superiorprocess control and product quality is positioned to expand marketshare.

[0008] A first problem is how to monitor the accumulation of a film onplasma tools inside a plasma process chamber. During an etch process,complex chemical processes take place, including the chemicaltransformation of photoresist on the surface of a wafer being etched,the removal of surface material via mechanical and/or chemicalprocesses. These processes create chemical species that deposit on thewalls of the chamber and other surfaces within the chamber. Thismaterial accumulates over the course of etching many wafers until itreaches a thickness at which the film under internal stresses breaks upand flakes off. These flakes can then move around the chamber, landingon the production substrate, leading to an immediate defect. Theseflakes can also land on other vital surfaces such as system probes,where they can adversely affect system performance.

[0009] One way to solve the same problem is to dismantle the processchamber and visually inspect the pieces. However, this is extremelyinefficient and costly because of the extended chamber downtime. Anotherway to solve the same problem is to use a spectrometer to look atemission of specific species. However, this can be more complicated andrequires adjustment when utilizing different plasma conditions.

[0010] Therefore, there is a need in the art to simply and efficientlymonitor the growth or accumulation of a film on plasma tools inside aplasma process chamber. Three patents disclose arrangements formonitoring the accumulation of films.

[0011] U.S. Pat. No. 6,146,492 (Cho et al), “Plasma process apparatuswith in situ monitoring, monitoring method, and in situ residue cleaningmethod,” appears to describe a plasma process apparatus and in situmonitoring method that is a complex approach having the disadvantagethat a reactive gas must be injected into a chamber after a process hasbeen completed, to allow the exiting gas to be analyzed to decipher thethickness of the wall film. A simpler approach allowing measuring offilm accumulation is needed.

[0012] U.S. Pat. No. 6,025,916 (Quick et al.), “Wall depositionthickness sensor for plasma processing chamber,” appears to describe adevice for measuring polymer build-up on plasma chamber walls thatinvolves the complex and indirect approach of monitoring interferencepatterns of light passing through a chamber window. A simpler and moredirect approach is desirable.

[0013] U.S. Pat. No. 5,948,983 (Gogol, Jr. et al.), “Wall depositionmonitoring system,” appears to describe a wall deposit monitoring systemfor measuring variation in wall deposit thickness in an etch ordeposition chamber. This complex method requires installation of apiezoelectric sensor on the chamber wall, and indirectly monitorscontaminants by measuring vibration damping created by filmaccumulation. Again, a simpler and more direct approach to measuringfilm accumulation is desirable.

[0014] A second problem is how to optimize the scheduling of plasmachamber cleaning. Maintaining a clean chamber in a plasma etch tool iscritical to producing integrated circuits (ICs) in a plasma etchingprocess. Production efficiency depends in particular on the cleanlinessof the process chamber. Conventional techniques for ensuring a cleanchamber include operating the plasma chamber for a predetermined timeand then dismantling the chamber for visual inspection. This techniquedoes not account for changing plasma conditions and does not result inan accurate real-time representation of the chamber cleanliness.Inaccurate representations of the chamber cleanliness can result inproduct defects. Therefore, it is necessary to properly schedulemaintenance to maximize production and maintain product quality.Therefore, there is a need in the art to optimize scheduling of plasmachamber cleaning.

[0015] A third problem is how to lower the cost of plasma etchprocesses. Maintenance and other non-productive time in a plasma etchsystem is extremely costly for manufacturers of semiconductor products.In addition, any defective product wastes valuable time and resources,increasing production costs. Therefore, there is a need in the art tolower the cost of plasma etch processes.

SUMMARY OF THE INVENTION

[0016] The present invention provides a method and apparatus formonitoring film build-up on the interior surfaces of a chamber such as aplasma process chamber.

[0017] A preferred embodiment of the invention provides an apparatus forsensing a degree of cleanliness of a plasma reactor having a chambercontaining a plasma that emits light during a process conducted in thechamber. The apparatus comprises a light sensing element configured tosense an intensity of the light emitted by the plasma after the lightpasses through a film that accrues in the chamber during plasmagenerating processes, and to provide a light intensity indicationsignal; and an electronics assembly configured to receive the lightintensity indication signal and to provide an indication of the degreeof cleanliness of the plasma reactor based on the light intensityindication signal.

[0018] Likewise, the invention provides a preferred method of sensing adegree of cleanliness of a plasma reactor having a chamber containing aplasma that emits light during a process conducted in the chamber. Themethod involves sensing an intensity of the light emitted by the plasmaafter the light passes through a film that accrues in the chamber duringthe process, and providing a light intensity indication signal, andproviding an indication of the degree of cleanliness of the plasmareactor based on the light intensity indication signal.

[0019] Alternate embodiments envision use of one or more light sourcesother than the plasma used in the semiconductor process itself, allowingtesting of film accrual before and after the semiconductor process isconducted.

[0020] Alternate embodiments also envision use of plural light sensorslocated at respective locations in the chamber, to sense accrual of filmat the respective locations.

[0021] A preferred method of monitoring a degree of accrual of a film inplasma chamber during a process conducted on semiconductor wafers in theplasma chamber, is also provided. The method involves loading asemiconductor wafer into the plasma chamber, starting the process on theloaded semiconductor wafer, and determining if the degree of accrual ofthe film has exceeded a threshold. If it is determined that the film hasexceeded the threshold, then an alarm is triggered and the process isfinished only for a current semiconductor wafer so as to allow amaintenance procedure to be performed on the chamber before the processis conducted on additional semiconductor wafers. If, however, it isdetermined that the film has not exceeded the threshold, then theprocess is completed and, if the process is to be conducted onadditional semiconductor wafers, the loading and starting steps arecarried out on the additional semiconductor wafers without firstperforming the maintenance procedure.

[0022] Thus, the present invention provides apparatus and methods tomonitor the growth or accumulation of a film on plasma tools insidechambers such as a plasma process chamber, and to assess plasma processchamber cleanliness in real-time, to optimize scheduling of plasmachamber cleaning, and thus strategically reduce contamination to theproduct substrate, thus improving product quality and yield and therebylowering the overall cost of processes such as plasma etch processes.

[0023] Other objects, features and advantages of the present inventionwill be apparent to those skilled in the art upon a reading of thisspecification including the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention is better understood by reading the followingDetailed Description of the Preferred Embodiments with reference to theaccompanying drawing figures, in which like reference numerals refer tolike elements throughout, and in which:

[0025]FIG. 1 shows a simplified schematic diagram of a plasma reactorillustrating various features of an exemplary embodiment of the presentinvention;

[0026]FIG. 2 illustrates a typical spectrum of emitted light, withwavelengths ranging from about 200 nm to about 900 nm (nanometers);

[0027]FIG. 3 shows a simplified schematic diagram of a plasma reactorillustrating various features of an alternate embodiment of theinvention;

[0028]FIG. 4 is a flow chart showing steps in an exemplary plasmachamber film monitoring method;

[0029]FIG. 5 illustrates a plot, as a function of etch-time, of a signalthat indicates the current of the photodiode, which in turn indicatesthe accrual of film in plasma reactor;

[0030]FIG. 6 illustrates a simplified schematic diagram of a ViewingAperture Assembly (VAA) in accordance with another embodiment of theinvention; and

[0031]FIG. 7 illustrates a simplified schematic diagram of a plasmaprocess tube in a plasma reactor illustrating various features of analternate embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] In describing preferred embodiments of the present inventionillustrated in the drawings, specific terminology is employed for thesake of clarity. However, the invention is not intended to be limited tothe specific terminology so selected, and it is to be understood thateach specific element includes all technical equivalents that operate ina similar manner to accomplish a similar purpose.

[0033]FIG. 1 shows a simplified schematic diagram of a plasma reactorillustrating various features of an exemplary embodiment of the presentinvention. In FIG. 1, a portion of a plasma reactor is shown along witha chamber wall deposition monitoring apparatus. Monitoring apparatus 100is used in conjunction with process chamber 102 that may be metal, and aprocess chamber wall 105 that includes at least one window 106 that issubstantially transparent to light of wavelength emitted by plasma inprocess chamber 102. Process chamber wall 105 surrounds process chamber102, and has an interior surface 130 on which film 135 grows duringprocesses such as semiconductor substrate etching processes ordeposition processes.

[0034] Only a portion of the process chamber and its walls are shown inFIG. 1 for purposes illustrating the invention's various features andnot to photographically represent an actual process chamber. Thus, it isunderstood that FIG. 1 is schematic in nature and not literal, so thatelements are not to scale.

[0035] As shown in FIG. 1, monitoring apparatus 100 comprises opticalguides 140 a and 140 b, optical coupler 150, light detector 180, andelectronics assembly 170. First optical guide 140 a and a second opticalguide 140 b are collectively referred to herein as element 140.

[0036] Optical guide 140 a has an optical entrance surface 141 andsecond optic end 142. Optical entrance surface 141 is located withinwindow 106 in process chamber wall 105. As shown in FIG. 1, window 106includes shoulder 107 and is mounted to a recess in chamber wall 105using sealing O-ring 108.

[0037] Optical coupler 150 comprises first optical connector 152 a andsecond optical connector 152 b. First optical guide 140 a is coupled tofirst optical connector 152 a, and second optical guide 140 b is coupledto second optical connector 152 b. First optical connector 152 acomprises first mating surface 154 a that mates with mating surface 154b on second optical connector 152 b, and the mating allows first opticalguide 140 a to be optically coupled to second optical guide 140 b.

[0038] First optical connector 152 a is coupled to chamber wall 105using fastener 158 and cooling O-ring 159. For example, mating surfaces154 a and 154 b can be threaded surfaces. In alternate embodiments,optical coupler 150 can include a light amplifier.

[0039] Second optical guide 140 b comprises optic end 144 and is alsooptically coupled to light detector 180. For example, light detector 180can be a commercially available silicon photodiode or similar type ofdiode having a wavelength detection range corresponding to thewavelength desired to be measured (in this example, 400 to 900nanometers). When the intensity of the light is sensed using aphotodiode, the light intensity indication signal is an electric currentinduced by light received by the photodiode.

[0040] Alternately, a unitary optical fiber (taking the place ofseparate elements) can be employed, and would not require the opticalconnectors between optic ends 142 and 144. However, splitting theoptical guide 140 into two components, namely component 140 a affixedwithin the chamber wall 105 and component 140 b to optically connect thechamber optical site to a remotely located light detector 180, providessimpler design, assembly and maintenance.

[0041] Plasma etch process chamber 102 may be used, for example, forsubstrate etching processes disposed within any plasma processing devicewhere excited species radiate energy. Such chambers include capacitivelycoupled plasma (CCP) and inductively coupled plasma (ICP) reactors,electron-cyclotron resonance (ECR) plasma sources, Helicon wave-heatedplasma sources, etc. Process chamber wall 105 includes interior surface130 on which film 135 accumulates.

[0042] Plasma 103 is created in the etch process chamber 102 accordingto process conditions including gas species present, power applied,temperature, etc., in accordance with principles known to those skilledin the art. Plasma 103 emits light 104. The intensity and emissionspectrum of the emitted light depends on the energy and chemicalcomposition of the plasma. In one example, plasma is formed using amixture of Ar, C₄F₈, O₂ and CO to achieve a plasma chemistry suitablefor oxide (i.e. SiO₂) etch. This plasma produces a low amplitude,relatively broadband spectrum (shown in FIG. 2) on which peaks aresuperimposed corresponding to the gas species present.

[0043] Film 135 is a deposition of a mixture of polymer particles andother byproducts that is formed while plasma etching a substrate. Thechemical composition of such films is not well understood by mostequipment manufacturers, since the chemical composition of photoresist(a key player in the film materials formed on chamber walls) istypically not disclosed. Film 135 has a thickness that depends onprocess conditions and the amount of time that the etching process hasbeen active.

[0044] In a preferred embodiment, optical guide 140 a comprises anoptical fiber having an optical entrance surface 141 and first optic end142. Alternately, optical guide 140 a can be a commercially availablequartz rod or waveguide through which the emitted light is capable ofbeing transmitted. Its dimensions may vary to correspond to the window'sdimensions and the area of light detector 160. For example, when windowthickness is approximately 10 mm, the optical fiber can have a diameter1 to 3 mm and a length of 100 to 150 mm. Similarly, optical guide 140 bcan be a commercially available optical fiber (or fiber optic bundle) ofdiameter 1 to 3 mm and length of 500 to 1500 mm.

[0045] Transmission media 165 comprises a suitable medium fortransmitting a signal that light detector 180 produces to indicate thequantity of light incident upon it, such as a shieldedelectrically-conductive wire.

[0046] Electronics assembly 170 includes a conventional arrangement of aCPU, memory and display collectively capable of processing and storingdata and interfacing with a user. Desirably, electronics assembly 170 iscapable of processing signals from light detector 180. For example,electronics assembly 170 is configured to receive consecutive lightintensity indication signals over time, and to trigger an alarm when alight intensity indication signal indicates that the cleanliness of theplasma reactor has declined beneath a threshold value of cleanliness.Also, electronics assembly performs operations described below infurther detail.

[0047] In the illustrated embodiment, window 106 is fabricated fromquartz (transmittance for 1 mm thick GE 214 quartz spans approximately180 to 4000 nm and transmittance for 1 cm thick GE 124 quartz spansapproximately 200 to 3500 nm). Alternatively, window 106 is fabricatedfrom alumina (transmittance for 2 mm thick crystalline or commercialgrade aluminum oxide spans 200 to 6000 nm).

[0048] Window 106 is oriented with respect to plasma and its emittedlight such that some of the emitted light passes through window 106,enters optical entrance surface 141, and is contained within firstoptical guide 140 a. First optical guide 140 a is positioned withinwindow 106 such that optical entrance surface 141 is in close proximityto surface 130. In alternate embodiments, optical entrance surface 141can be embedded within window 106.

[0049] Film 135 forms on exposed surfaces of process chamber wall 105,in particular on interior surface 130 and on window 106. Film 135 is themain mechanism by which light is attenuated before entering opticalentrance surface 141. The amount of attenuation caused by window 106 isreduced by placing optical entrance surface 141 within recess 109 withinwindow 106, thereby minimizing the effective thickness of the window 106in the area immediately adjacent optical entrance surface 141.

[0050] In operation, upon formation of plasma, the plasma radiatesemitted light, a portion of the emitted light passing through film 135to be attenuated to a degree determined by the thickness of film 135.The attenuated emitted light passes through window 106 and is incidentupon optical entrance surface 141. The attenuated incident light entersoptical guide 140 a and passes through optical guides 140 a, 140 b,until it impinges upon light detector 180. Light detector 180 transformsthe portion of the attenuated emitted light it receives into an electricsignal that travels via transmission media 165 to electronics assembly170. Electronics assembly 170 stores and processes the signal, andpresents a user with a suitable graphic, audible, and/or numeric displayand/or alarm element(s) representing the state of cleanliness of theprocess chamber wall.

[0051] However, over time, plasma processes performed in chamber 102cause film 135 to accumulate on interior surface 130 of process chamberwall 105 and window 106. As film 135 accumulates on interior surface 130of process chamber wall 105 and window 106, a decreasing amount ofemitted light is able to reach optical guide 140 a. Over time,accumulated film 135 thus reduces the amount of emitted light thattravels via optical guide 140 b and reaches light detector 180. As aresult of the film accumulation, light detector 180 sends progressivelysmaller signals via transmission media 165 to electronics assembly 170,which processes these signals and reports the increasing filmaccumulation to the user.

[0052] In alternative embodiments of the present invention, multipleoptical sites with respective windows and optical entrance surfaces canbe located in plural locations within the process chamber 102, alongwith corresponding optical guides and light detectors 180 that can senda plurality of signals to the electronics assembly 170. Electronicsassembly 170 can process the plurality of signals and can report filmaccumulation to the user.

[0053]FIG. 3 shows a simplified schematic diagram of a plasma reactorillustrating various features of an alternate embodiment of theinvention. In FIG. 3, a portion of the plasma reactor is shown alongwith an exemplary optical source 300. Optical source 300 is used inconjunction with process chamber 302 that may be metal, and processchamber wall 305 that includes at least one window 306 that issubstantially transparent to light of wavelength emitted by opticalsource 300. Process chamber wall 305 surrounds process chamber 302, andhas an interior surface 330 on which a film 335 grows during processessuch as semiconductor substrate etching processes.

[0054] Only a portion of the process chamber and its walls are shown inFIG. 3 for purposes illustrating the invention's various features andnot to photographically represent an actual process chamber. Thus, it isunderstood that FIG. 3 is schematic in nature and not literal so thatelements are not to scale.

[0055] In an alternate embodiment, the monitoring apparatus (FIG. 1.)can include at least one optical source as shown in FIG. 3.

[0056] In the illustrated embodiment, optical source 300 comprisesoptical guides 340 a and 340 b, optical coupler 350, light source 380,and electronics assembly 370. First optical guide 340 a and a secondoptical guide 340 b that are collectively referred to herein as element340.

[0057] First optical guide 340 a comprises optical output surface 341,and second optic end 342. Optical output surface 341 is located withinwindow 306 in process chamber wall 305. As shown in FIG. 3, window 306includes shoulder 307 and is mounted to a recess in chamber wall 305using sealing O-ring 308.

[0058] Optical coupler 350 comprises first optical connector 352 a andsecond optical connector 352 b. First optical guide 340 a is coupled tofirst optical connector 352 a, and second optical guide 340 b is coupledto second optical connector 352 b. First optical connector 352 acomprises first mating surface 354 a that mates with mating surface 354b on second optical connector 352 b, and the mating allows first opticalguide 340 a to be optically coupled to second optical guide 340 b.

[0059] First optical connector is coupled to chamber wall 305 usingfastener 358 and cooling O-ring 359. For example, mating surfaces 354 aand 354 b can be snap-together surfaces. Alternately, optical coupler350 can include a light amplifier.

[0060] Second optical guide 340 b comprises optic end 344 and is alsooptically coupled to light source 380. For example, light source 380 isa semiconductor device.

[0061] Light source 380 is coupled to electronics assembly 370 usingtransmission medium 365. For example, light source 380 can be acommercially available light emitting device having a wavelength outputrange corresponding to the wavelength desired to be measured (in thisexample, 400 to 900 nanometers).

[0062] Film 335 is a deposition of a mixture of polymer particles andother byproducts that is formed while plasma etching a substrate. Thechemical composition of such films is not well understood by mostequipment manufacturers, since the chemical composition of photoresist(a key player in the film materials formed on chamber walls) istypically not disclosed. Film 335 has a thickness that depends onprocess conditions and the amount of time that the etching process hasbeen active.

[0063] For example, optical guide 340 a can be an optical fiber, or itcan be a commercially available quartz rod or waveguide through whichthe emitted light is capable of being transmitted. Similarly, opticalguide 340 b can be a commercially available optical fiber (or fiberoptic bundle) of diameter 1 to 3 mm and length of 500 to 1500 mm.

[0064] Transmission media 365 comprises a suitable medium forestablishing an interface between electronics assembly 370 and lightsource 380. The interface being used to convey signals for determining,for example, the amount of light that light source 380 produces at aparticular time.

[0065] Electronics assembly 370 includes a conventional arrangement of aCPU, memory and display collectively capable of processing and storingdata and interfacing with a user as described earlier in reference toelectronics assembly (170 FIG. 1) and further being capable ofcontrolling light source 380.

[0066] Window 306 can be fabricated from quartz (transmittance for 1 mmthick GE 214 quartz spans approximately 180 to 4000 nm and transmittancefor 1 cm thick GE 124 quartz spans approximately 200 to 3500 nm), oralumina (transmittance for 2 mm thick crystalline or commercial gradealuminum oxide spans 200 to 6000 nm).

[0067] For example, first optical guide 340 a can be oriented withrespect to first optical guide (140 a FIG. 1) such that some of theemitted light at optical output surface 341 passes through the processchamber and enters optical entrance surface (141 FIG. 1) and is measuredby a monitoring system as described above.

[0068] Window 306 is relatively transparent to light emitted by opticalsource 300. Film 335 forms on exposed surfaces of process chamber wall305, in particular on interior surface 330 and on window 306. Film 335is the main mechanism by which light from optical source 300 isattenuated before entering the chamber. The amount of attenuation causedby window 306 can be reduced by placing optical output surface 341within recess 309, thereby minimizing the effective thickness of thewindow 306 in the area immediately adjacent optical output surface 341.Alternately, a unitary optical guide (taking the place of separateelements) can be employed. For example, splitting the optical guide intotwo components provides simpler design, assembly and maintenance.

[0069] In operation, upon command from electronics assembly 370, opticalsource 300 emits light, a portion of the emitted light passing throughtwo layers of film 335 and being measured by a monitoring system asdescribed above. The monitoring system uses the amount of attenuation todetermine the thickness of the films.

[0070]FIG. 4 is a flow chart illustrating an exemplary plasma chamberfilm monitoring method according to the present invention.

[0071] After the start of the method, in step 405 the chamber is cleanedusing manufacturer recommended methods along with cleaning steps toremove film 135 from interior surface 130 of process chamber wall 105.Typically, the chamber may be cleaned using an oxygen plasma or thechamber liner and components may be removed for a wet clean andreplaced. At this time, electronics 170 may be calibrated to provide areference to which changes in the signal created by light detector 180can be compared. The calibration process of step 405 includes measuringthe light detector response under process conditions to be used. If morethan one process recipe is used in the reactor, then the response oflight detector 180 for each process recipe is obtained to create acalibration chart. Moreover, the system may be calibrated using theexternal light source as described above. The calibration chart showslight intensities as a function of wall film thickness for every processscenario. Because the relationship is between light intensity and filmthickness is known after experimentation, only a limited number of datapoints is needed to generate each correlation chart.

[0072] In step 410, a production wafer is loaded into process chamber102 and placed on a wafer chuck (not shown), and the wafer etchingprocess proceeds according to the process recipe selected for the wafer.

[0073] In decision step 415, electronics assembly 170 performs acomparison that determines whether the chamber is clean enough tocontinue normal process operation. As described above, light detector180 converts the emitted light it receives into an electric signal,which is normally a value of electric current that is sensed byelectronics 170. Electronics assembly 170 receives and stores theelectric signal. Electronics assembly 170 then compares the value of thecurrent to a threshold value 556 (see FIG. 5) determined by thecalibration chart and the user. If threshold value 556 has been met orexceeded, then the system continues to step 220. If threshold value 556has not been met or exceeded, then the system diverts to step 440,discussed below.

[0074] In step 420, the wafer continues to be etched within processchamber 102.

[0075] In decision step 425, a comparison is made to see if the waferetching process has been completed. If the etching process has not beencompleted, then control is returned to step 415. If the etching processhas been completed, then control passes to step 430. In step 430, theplasma etching process on one wafer terminates.

[0076] In step 435, if it is determined that there are more wafers to beprocessed, the control returns to step 410. If it is determined thatthere are no more wafers to be processed, then method ends, indicated byblock 455.

[0077] In step 440, which is executed after decision block 415determined that the chamber was not clean enough, electronics assembly170 activates an indication or alarm alerting the user that film 135 hasreached threshold response 557 (FIG. 5) and thus indicates processchamber wall 105 no longer meets the cleanliness standards defined bythe calibration chart.

[0078] In step 445, the etching process of the current wafer iscompleted, and in step 450, a pre-determined maintenance protocol isinitiated for the specific process that was completed in step 445.Finally, method 400 terminates, as indicated by block 455.

[0079]FIG. 5 shows a plot of received signal versus etch-time. Receivedsignal 552 is shown on the vertical axis, and etch-time 553 on thehorizontal axis. A response curve 558 shows the relation of the receivedsignal 552 and etch time 553, an initial value 554 of signal 552 at atime offset 555, a threshold value 556 of the signal 552 correspondingto a threshold response 557 as determined by response curve 558.

[0080] Signal versus etch-time plot 551 plots signal 552 versusetch-time 553 as response 558. Signal 552 is created by electronicsassembly 170, and is based on a sensed parameter such as photodiodecurrent. Offset 555 is the time in which response 558 settles to becomea decipherable signal with an initial value 554. Response 558 thendecays exponentially until reaching threshold value 556, whichcorresponds to threshold response 557.

[0081] Response 558 represents many data points that correspond to oneor more measurements on individual wafers and measurements on one ormore wafers. That is, it is expected that the reduction in signal 552while etching any single wafer is slight. Below threshold value 556,noise in the form of uncontrolled changes in response 558 are due inpart to flakes of film dislodging from a window, and, conversely, flakesof film dislodging from surfaces within the process chamber adhering tothe window. Threshold response 557, which may be affected by systemparameters such as plasma power, plasma gas composition and substratematerials, may be determined through experimentation by those skilled inthe art.

[0082]FIG. 6 illustrates a simplified schematic diagram of a ViewingAperture Assembly (VAA) in accordance with another embodiment of theinvention. VAA 600 is shown mounted in a process chamber wall 610. VAA600 comprises tip 620 that is coupled to housing 625. For example, aceramic tip can be brazed to a metal housing. Tip 620 can be fabricatedfrom alumina or like ceramic material. Housing 625 is fabricated fromstainless steel. The housing is threaded and is installed in anappropriate hole produced in the process chamber to receive the threadedfeature. O-ring 630 seals the VAA as shown. A number of VAAs can bepositioned in various locations around a chamber wall. Light passingthrough the VAA and the associated fiber to a photodiode is used tocreate a signal that represents the cleanliness of the reactor. Thephotodiode detects the intensity of the light and the apparatus comparesthe intensity values to a chart of known intensities for processconditions and film thickness. As film build-up increases on the window,the light intensity decreases.

[0083] Small cylindrical light passage 640 is located in the center ofthe housing and couples tip 620 to optical connector 635. Opticalconnector 635 is located at the end of the housing opposite the ceramictip and is manufactured in a manner to produce features that can mate tooptical cables attached to a photodiode assembly. The threaded joint isvacuum tight to a high vacuum level. VAA 600 provides an inexpensivereplaceable monitoring assembly that functions as a sealed window in aprocess chamber and can be used in a monitoring system described herein.In alternate embodiments, VAA 600 can further comprise a light detectorand/or a light source.

[0084]FIG. 7 illustrates a simplified schematic diagram of a plasmaprocess tube in a plasma reactor illustrating various features of analternate embodiment of the invention. In FIG. 7, a portion of a plasmareactor is shown along with a process tube based deposition monitoringapparatus. Monitoring apparatus 700 is used in conjunction with processtube that is mounted within process chamber 702. Process chamber cancomprise metal, and process tube can comprise a material that issubstantially transparent to light of wavelength emitted by plasma inprocess chamber 702. Process tube wall 710 surrounds process chamber702, and has an interior surface 730 on which film 735 grows duringprocesses such as semiconductor substrate etching processes ordeposition processes.

[0085] Only a portion of the process chamber, the process tube, andtheir respective walls are shown in FIG. 7 for purposes illustrating theinvention's various features and not to photographically represent anactual process chamber. Thus, it is understood that FIG. 7 is schematicin nature and not literal, so that elements are not to scale.

[0086] As shown in FIG. 7, monitoring apparatus 700 comprises opticalguides 740 a and 740 b, optical coupler 750, light detector 780, andelectronics assembly 770. First optical guide 740 a and a second opticalguide 740 b are collectively referred to herein as element 740.

[0087] First optical guide 740 a comprises optical output surface 741,and second optic end 742. Optical output surface 741 is located withinrecess 706 in the flange portion of process tube wall 710. As shown inFIG. 7, process tube wall 710 includes flange 715 and is mounted to arecess in chamber wall 705 using O-ring 708.

[0088] Optical coupler 750 comprises first optical connector 752 a andsecond optical connector 752 b. First optical guide 740 a is coupled tofirst optical connector 752 a, and second optical guide 740 b is coupledto second optical connector 752 b. First optical connector 752 acomprises first mating surface 754 a that mates with mating surface 754b on second optical connector 752 b, and the mating allows first opticalguide 740 a to be optically coupled to second optical guide 740 b.

[0089] First optical connector is coupled to chamber wall 705 usingfastener 758. For example, mating surfaces 354 a and 354 b can besnap-together surfaces. Alternately, optical coupler 750 can include alight amplifier.

[0090] Second optical guide 740 b comprises optic end 744 and is alsooptically coupled to light detector 780.

[0091] Film 735 is a deposition of a mixture of polymer particles andother byproducts that is formed while plasma etching a substrate. Film735 has a thickness that depends on process conditions and the amount oftime that the etching process has been active.

[0092] For example, optical guide 740 a can be an optical fiber, or itcan be a commercially available quartz rod or waveguide through whichthe emitted light is capable of being transmitted. Similarly, opticalguide 740 b can be a commercially available optical fiber (or fiberoptic bundle) of diameter 1 to 3 mm and length of 500 to 1500 mm.

[0093] Transmission media 765 comprises a suitable medium forestablishing an interface between electronics assembly 770 and lightsource 780. The interface being used to convey signals for determining,for example, the amount of light that light detector 780 detects at aparticular time.

[0094] Electronics assembly 370 includes a conventional arrangement of aCPU, memory and display collectively capable of processing and storingdata and interfacing with a user as described earlier in reference toelectronics assembly (170 FIG. 1) and further being capable ofcontrolling light detector 780.

[0095] Process tube wall 710 can be fabricated from quartz(transmittance for 1 mm thick GE 214 quartz spans approximately 180 to4000 nm and transmittance for 1 cm thick GE 124 quartz spansapproximately 200 to 3500 nm), or alumina (transmittance for 2 mm thickcrystalline or commercial grade aluminum oxide spans 200 to 6000 nm).

[0096] Process tube wall 710 is relatively transparent to light emittedby the plasma. Film 735 forms on exposed surfaces of process tube wall710, in particular on interior surface 730. Film 735 is the mainmechanism by which light 704 emitted from plasma 703 is attenuatedbefore entering recess 706. The amount of attenuation caused by processtube wall 710 can be reduced by placing optical output surface 741within recess 706, thereby minimizing the effective thickness of thetube wall 710 in the area immediately adjacent optical output surface741.

[0097] In alternative embodiments of the present invention, multipleoptical sources can be used and can be located in plural locationsaround the process chamber, along with corresponding monitoring devices.

[0098] In other embodiments, one or more light sources can be providedinside the plasma chamber, directed at interior surface of processchamber wall to provide constant intensity light in lieu ofplasma-emitted light. For example, electronics assembly assemblesinformation from the one or more light sources and reports the degree ofcleanliness of the chamber, possibly providing an alarm indication ifcleanliness parameters (determined by experimentation and calibration)are violated.

[0099] Of course, the several monitoring systems and optical sources maybe arranged and/or used in various combinations. Thus, it is notnecessary to arrange an external light source 380 only with an opticaloutput surface of type 341, and it is not necessary to arrange themonitoring device only with an optical entrance surface of type 141,since these are merely examples for purposes of explanation. It isemphasized that the arrangements of elements in FIG. 1 and FIG. 3 areillustrative and do not limit the invention.

[0100] As a further feature, a light source, which may be an externallight source, can be employed when the plasma is turned off to couplelight onto interior surfaces of the process chamber to illuminate theinterior volume in order to check/calibrate wall monitoring sites fromwafer-to-wafer, between cleaning cycles, etc.

[0101] Modifications and variations of the above-described embodimentsof the present invention are possible, as appreciated by those skilledin the art in light of the above teachings. For example, varying thelocation and number of optical entrance surfaces, optical paths, andlight detecting elements lies within the contemplation of the presentinvention. Also, the particular light source used to generate light thatis passed through the film for attenuation, the particular way in whichattenuated light is transmitted to a light sensor, the particular mannerin which light intensity is detected and communicated, and theparticular way in which the detected light level is communicated,stored, processed and reported to a user, may be varied while remainingwithin the scope of the invention. It is therefore to be understoodthat, within the scope of the appended claims and their equivalents, theinvention may be practiced otherwise than as specifically described.

1. An apparatus for sensing a degree of cleanliness of a plasma reactorhaving a chamber containing a plasma that emits light during a processconducted in the chamber, the apparatus comprising: a light sensingelement, configured to sense an intensity of the light emitted by theplasma after the light passes through a film that accrues in the chamberduring the process, and to provide a light intensity indication signal;and an electronics assembly configured to receive the light intensityindication signal and to provide an indication of the degree ofcleanliness of the plasma reactor based on the light intensityindication signal.
 2. The apparatus of claim 1, wherein: the lightsensing element is a photodiode; and the light intensity indicationsignal is an electric current induced by light received by thephotodiode.
 3. The apparatus of claim 1, further comprising: an opticalpath extending from an interior surface of the chamber to the lightsensing element, the optical path including an optical entrance surfacelocated behind the accrued film so as to receive emitted light that hasbeen diminished in intensity by being passed through the film.
 4. Theapparatus of claim 1, wherein the electronics assembly is configured toreceive consecutive light intensity indication signals over time, and totrigger an alarm when a light intensity indication signal indicates thatthe cleanliness of the plasma reactor has declined beneath a thresholdvalue of cleanliness.
 5. The apparatus of claim 1, wherein: the processis a semiconductor etching process during which the film accrues onwalls of the chamber.
 6. The apparatus of claim 1, further comprising: asecond light sensing element, configured to sense a second intensity ofthe light emitted by the plasma after the light passes through a filmthat accrues at a second location in the chamber, and to provide asecond light intensity indication signal indicative of the degree offilm accrual at the second location.
 7. An apparatus for sensing adegree of cleanliness of a chamber in which a process is conducted, theapparatus comprising: a light emitting element configured to emit light;a light sensing element, configured to sense an intensity of the emittedlight after the light passes through a film that accrues in the chamberduring the process, and to provide a light intensity indication signal;and an electronics assembly configured to receive the light intensityindication signal and to provide an indication of the degree ofcleanliness of the chamber based on the light intensity indicationsignal.
 8. The apparatus of claim 7, wherein the light emitting elementis disposed within the chamber.
 9. The apparatus of claim 7, wherein thelight emitting element is disposed outside the chamber and emits lightthrough a wall of the chamber so that a portion of the emitted lightenters the light sensing element.
 10. The apparatus of claim 7, furthercomprising: a second light sensing element configured to sense a secondlight intensity of light that passes through film that accrues at asecond location in the chamber, and to provide a second light intensityindication signal indicative of the degree of film accrual at the secondlocation.
 11. A method of sensing a degree of cleanliness of a plasmareactor having a chamber containing a plasma that emits light during aprocess conducted in the chamber, the method comprising: sensing anintensity of the light emitted by the plasma after the light passesthrough a film that accrues in the chamber during the process, andproviding a light intensity indication signal; and providing anindication of the degree of cleanliness of the plasma reactor based onthe light intensity indication signal.
 12. The method of claim 11,wherein: the sensing step includes sensing the intensity of the lightusing a photodiode; and the light intensity indication signal is anelectric current induced by light received by the photodiode.
 13. Themethod of claim 11, further comprising: carrying some of the lightemitted by the plasma along an optical path extending from an interiorsurface of the chamber to the light sensing element, the optical pathincluding an optical entrance surface located behind the accrued film soas to receive emitted light that has been diminished in intensity bybeing passed through the film.
 14. The method of claim 11, furthercomprising: receiving consecutive light intensity indication signalsover time, and triggering an alarm when a light intensity indicationsignal indicates that the cleanliness of the plasma reactor has declinedbeneath a threshold value of cleanliness.
 15. The method of claim 11,wherein: the process is a semiconductor etching process during which thefilm accrues on walls of the chamber.
 16. A method of sensing a degreeof cleanliness of a chamber during a process conducted in the chamber,the method comprising: emitting light into the chamber; sensing anintensity of the emitted light after the emitted light passes through afilm that accrues in the chamber during the process, and providing alight intensity indication signal; and providing an indication of thedegree of cleanliness of the chamber based on the light intensityindication signal.
 17. The method of claim 16, wherein the emitting stepincludes: emitting the light into the chamber with a light emittingelement that is disposed within the chamber.
 18. The method of claim 16,wherein the emitting step includes: with a light emitting element thatis disposed outside the chamber, emitting the light through a wall ofthe chamber so that a portion of the emitted light is sensed in thelight intensity sensing step.
 19. The method of claim 16, furthercomprising: sensing a second intensity of light that passes through filmthat accrues at a second location in the chamber, and providing a secondlight intensity indication signal indicative of the degree of filmaccrual at the second location.
 20. A method of monitoring a degree ofaccrual of a film in a chamber in which a process is conducted onsemiconductor wafers, the method comprising: loading a semiconductorwafer into the plasma chamber; starting the process on the loadedsemiconductor wafer; determining if the degree of accrual of the filmhas exceeded a threshold; if it is determined that the film has exceededthe threshold, then triggering an alarm and finishing the process onlyfor a current semiconductor wafer so as to allow a maintenance procedureto be performed on the chamber before the process is conducted onadditional semiconductor wafers; and if it is determined that the filmhas not exceeded the threshold, then completing the process and, if theprocess is to be conducted on additional semiconductor wafers, carryingout the loading and starting steps on the additional semiconductorwafers without first performing the maintenance procedure.
 21. Themethod of claim 20, wherein the film accrual degree determining stepincludes: sensing an intensity of emitted light after the emitted lightpasses through the film that has accrued in the chamber during theprocess, and providing a light intensity indication signal that isindicative of the degree of accrual of the film.
 22. The method of claim21, wherein: plasma in the chamber emits light during the processconducted on the semiconductor wafers; and the emitted light intensitysensing step includes sensing the intensity of the light emitted by theplasma after the emitted light passes through the film that has accruedin the chamber.
 23. The method of claim 21, wherein: a light emittingelement disposed within the chamber emits light; and the emitted lightintensity sensing step includes sensing the intensity of the lightemitted by the light emitting element after the emitted light passesthrough the film that has accrued in the chamber.
 24. The method ofclaim 21, wherein: a light emitting element disposed outside the chamberemits light into the chamber; and the emitted light intensity sensingstep includes sensing the intensity of the light emitted by the lightemitting element after the emitted light passes through the film thathas accrued in the chamber.
 25. The method of claim 21, wherein: theemitted light intensity sensing step includes sensing the intensity ofthe light emitted at plural locations in the chamber after the emittedlight passes through film that has accrued at the plural locations inthe chamber.
 26. A viewing aperture assembly (VAA) for sensing a degreeof cleanliness of a plasma reactor having a chamber containing a sourcethat emits light during a process conducted in the chamber, the VAAcomprising: a threaded housing including O-ring for coupling to athreaded opening in a chamber wall; a transparent tip coupled to thethreaded housing; an optical connector coupled to the threaded housing;and a light passage coupled to the transparent tip and the opticalconnector, the VAA being configured to sense an intensity of the lightemitted by the source after the light passes through a film that accruesin the chamber during the process, and to provide a light intensityindication signal.
 27. The apparatus of claim 26, further comprising: alight sensing element configured to sense a light intensity of lightthat passes through film that accrues at a location in the chamber, andto provide a light intensity indication signal indicative of the degreeof film accrual at the second location.
 28. The apparatus of claim 26,further comprising: a light emitting element configured to emit lightinto the chamber.
 29. An apparatus for sensing a degree of cleanlinessof a plasma reactor having a process tube containing a plasma that emitslight during a process conducted in the process tube, the apparatuscomprising: a light sensing element, configured to sense an intensity ofthe light emitted by the plasma after the light passes through a filmthat accrues in the process tube during the process, and to provide alight intensity indication signal; and an electronics assemblyconfigured to receive the light intensity indication signal and toprovide an indication of the degree of cleanliness of the plasma reactorbased on the light intensity indication signal.